In order to achieve a high degree of reliability in extensive telecommunication systems, it is normal for functional units in the system switch to utilize several, mutually-parallel planes, each containing mutually-identical equipment. As used throughout this application, the terms "mutually-identical" and "electrically identical" equipment mean electronic circuitry configures such that when an electrical signal is input to two or more pieces of such equipment, when functioning without error, the outputs from such equipment will be the same. The terms "equipment" and "processing means" include but are not limited to processors, memories and/or other electronic circuitry. The data-flow entering such a multi-plane unit is distributed to the parallel planes contained therein. The data-flow passes through the equipment in each of said planes and an output data signal is then delivered from respective planes. The occurrence of an error, or a fault, in any one of the planes can be discovered, by comparing the data signals delivered by the various planes, bit for bit, and a majority-selection of data-flow can be delivered from the unit by making a majority selection on the basis of said comparison. The comparison and the majority-selection are effected in a comparator/majority-selection device connected to the multi-plane unit.
As before mentioned, the comparator/majority-selection device is operative to make a comparison between the data signals exiting from the planes, bit-for-bit in real time, and delivers an output signal which constitutes a majority decision. In order for this to be possible, the output data signals must be delivered from the parallel planes synchronously, i.e. the delays through the three planes must be of mutually equal value, since otherwise the comparator/majority-selection device will produce an erroneous signal.
Swedish Patent Application SE 8705065-4 describes a time switch which is based on three synchronously operating planes and which operates with bit-for-bit comparison and majority-selection between corresponding information deriving from the three planes. The comparison and majority-selection are utilized to correct errors/faults in information occurring in any one plane in the switch. Each plane includes switch storage devices SSA, SSB, SSC, control storage devices CSA, CSB, CSC, re-write devices RWA, RWB, RWC and information selecting devices MD1A . . . MD4C. Each information selector device is operative to make a comparison between the information from the three planes and to make a majority-selection on the basis of this comparison, therewith correcting any error/fault that may occur in any one of the three planes.
Japanese Patent Specification JP 60-10996 describes a time selector which has time selecting devices in three planes. Each plane includes selector storage devices 1a, 1b, 1c and control storage devices 3a, 3b, 3c. A majority-selection is made from the connection information from the three planes in a majority selector device 8 and the majority-selected connection information is delivered to the selector storage devices 1a, 1b, 1c. A majority selection is made on information from the selector storage devices 1a, 1b, 1c in a majority-selection device 7, and delivered from the selector storage devices 1a, 1b, 1c on an output line 5.
British Patent Specification GB 1570 113 describes an invention relating to electronic systems which include equipment in parallel planes. The equipment operates with majority-selection, so as to obtain correct output signals even though an error/fault might occur. The object of the invention is to provide an arrangement for localizing errors/faults in the system. The error/fault localization is program-controlled and functions on the concept of testing one unit at a time. The selection information comes from a programmable read only memory (PROM). Each plane includes a device WSU for wave shape selection, these devices WSU being operative to cause the unit selected for testing to carry out a given operation so that a waveshape corresponding to said operation is formed. The generated waveshapes produced in the parallel planes in said unit are sent to a comparator CCU, in which they are compared, therewith enabling the detection of an error/fault in any one of the planes.